1. Field of the Invention
The invention relates to a patterning method, and particularly, to an ink printing apparatus for compensating mis-alignment of patterns caused by variation of a substrate by compensating the printing of the substrate when the length of the substrate has changed.
2. Description of the Background Art
Display devices, especially flat panel displays such as a liquid crystal display (LCD) are operated by switching an active device such as a thin film transistor (TFT) on the respective pixels. This fashion of switching a display device is called an active matrix operating method. In this active matrix operating method, the active devices are disposed on respective pixels, which are arranged in a matrix form to operate the corresponding pixels.
FIG. 1 shows an active matrix type liquid crystal display device. The liquid crystal display device shown in FIG. 1 is a thin film transistor (TFT) LCD using a thin film transistor as the active device. Each respective pixel of the TFT LCD, on which N×M pixels are disposed in transverse and longitudinal directions, has a gate line 4 through which a scan signal is applied from an outer operational circuit, a data line 6 through which an image signal is applied, and a TFT formed on a crossed area of the gate line 4 and the data line 6. The TFT has a gate electrode 3 connected to the gate line 4, a semiconductor layer 8 formed on the gate electrode 3 and activated according to application of the scan signal to the gate electrode 3, and a source/drain electrode 5 formed on the semiconductor layer 8. On a display area of the pixel 1, a pixel electrode 10 connects to the source/drain electrode 5 for switching a liquid crystal (not shown) by applying an image signal through the source/drain electrode 5 by activating the semiconductor layer 8.
The source/drain electrode 5 of the TFT is electrically connected to the pixel electrode 10 formed in the pixel 1, and displays an image by activating the liquid crystal according the signal being applied to the pixel electrode 10 through the source/drain electrode 5.
In an active matrix type display device such as the LCD described above, the pixel has a size on the order of tens of μm. Therefore, the active device such as a TFT disposed in the pixel should be formed to be a few μm. Moreover, as demand for display devices of superior image quality, such as superior image quality high definition TV (HDTV), gradually increases, a greater concentration of pixels are disposed on a screen of same area. Therefore, the active device patterns (including gate line and data line patterns) in the pixel become finer, i.e., smaller in size.
On the other hand, in the conventional fabrication of an active device such as a TFT, patterns or lines of the active device are formed using a photolithographic method requiring an exposure apparatus. However, the exposure apparatus is very expensive so that the resultant fabrication cost increases, and the fabrication process becomes complex. Moreover, the exposure area of the exposure apparatus is limited in photolithographic fabrication of the display device, and the photolithographic process is separately piecemeal performed, after dividing the screen, to fabricate a display device of larger area. Therefore, it is difficult to match the divided areas at a precise location during the piecemeal processing of the divided areas. As a result, productivity lowers due to the numerous repetitions of the photolithographic process.
In order to address the above problems, a method for patterning using gravure offset printing has been recently suggested. The gravure printing method is a printing method which stains an engraved plate with ink, and excess ink is scraped off. Gravure offset printing is used in various fields, including for publishing, for printing on packaging, for printing on cellophane, for printing on vinyl, and for printing on polyethylene. Recent research has attempted to apply the gravure printing method to the manufacture of an active device or to a circuit in the fabrication of a display device.
In the gravure offset printing method, the ink is transferred to the substrate using a transfer roll. Therefore, a larger area display device can be patterned by using a transfer roll corresponding to the size of the display device. Gravure offset printing can be used for patterning various patterns of the display device, for example, the gate line and data line connected to the TFT, the pixel electrode, the metal pattern for the capacitor, and the TFT in the LCD.
FIG. 2 shows a conventional art method of patterning using gravure offset printing.
As shown in FIG. 2A, a recess 22 is formed at a certain position on an engraved plate or on a cliché 20 corresponding to a pattern which will be formed on the substrate. Ink 24 fills the recess 22. The filling of ink 24 into the recess 22 is made by applying the patterning ink 24 for on an upper part of the cliché 20. Then, a doctor blade 28 contacts the cliché 20 to remove excess ink. The ink 24 fills the interior of the recess 22 by the action of the doctor blade 28, and the ink 24 remaining on the surface of the cliché 20 is simultaneously removed.
As shown in FIG. 2B, the ink 24 filling the recess 22 of the cliché 20 is transferred to a surface of a transfer roll 30, which rotates to contact the surface of the cliché 20. The transfer roll 30 is constructed to have the same width as that of the panel of the display device to be fabricated. The transfer roll 30 also has the same diameter as the length of the panel. Therefore, the ink 24 filled in the recess 22 of the cliché 20 is completely transferred onto the circumferential surface of the transfer roll 30 by rotation. After that, as shown in FIG. 2C, the transfer roll 30 is rotated in contact with a processed layer 41 formed on the substrate 40, and the ink 24 transferred on the transfer roll 30 is re-transferred onto the processed layer 41. The re-transferred ink 24 is dried by heating to form an ink pattern 42. At this time, the desired ink pattern 42 can be formed on an entire substrate 40 of the display device using only a single rotation of the transfer roll 30.
FIG. 3 shows a conventional art method of patterning the panel of a display device using the gravure offset printing. As shown in FIG. 3, the cliché 20, in which the ink 24 is filled in the recess 22 thereof, and the substrate 40 are located in the same plane. Although it is not shown in FIG. 3, the cliché 20 and the substrate 40 are affixed to a plate for ink printing, and the transfer roller 30 is installed on a side surface of the plate.
In the above-described printing apparatus, transfer roll 30 rotates and proceeds over the cliché 20. The ink 24 transfers to the circumferential surface of the transfer roll 30. In addition, as the transfer roll 30 continues to rotate and proceeds over the substrate 40, the ink 24 is re-transferred to the substrate 40 to form the ink pattern 42. In the above-described gravure offset printing method, the substrate 40 has substantially the same size as that of the cliché 20, and the substrate 40 is disposed on same plane as that of the cliché 20. The transfer roll 30 is rotated and proceeds from the cliché 20 toward and over the substrate 40 to print the ink pattern. Therefore, the ink pattern can be formed on the substrate having a larger area through simple processing, and the pattern can be formed on a larger area substrate such as a liquid crystal panel by post-processing.
However, the method for patterning using gravure offset printing has some problems. Generally, in order to form the pattern of the display device such as an LCD, the above-described gravure offset printing processes repeats a number of times. Various patterns, for example, the gate line or the gate electrode and the data line, are located on different planes. Therefore, in order to form the gate line and the data line, the printing process must be repeated on their respective planes. In addition, in order to form a metal pattern such as for the gate line and the data line, the printing process should be made after a metal layer is laminated by a sputtering or evaporation process. However, the sputtering or evaporation process is usually conducted at high temperature, and therefore the substrate 40 may expand or contract by the heat processing or cooling processing.
Generally, in the gravure offset printing method, the area of the substrate on which the pattern will be formed is set as nearly identical as the area of the cliché 20 and the circumferential surface of the transfer roll 30. Therefore, the ink pattern 42 is formed on entire substrate 40 by one printing process. However, when the substrate contracts or expands from thermal cycling, the area of the substrate 40 deviates from the area of the cliché 20 or the circumferential surface of the transfer roll 30.
FIG. 4 shows the substrate 40 expanded Δx toward the x direction. The substrate 40 enlarges as much as Δx greater than the cliché 20 in which the ink 24 is filled. Consequently, the areas of cliché 20 and of the substrate 40 differ from each other, and therefore it is impossible to accurately transfer the ink in the cliché 20 onto the substrate 40 in its actual intended form.
Hereinafter, problems of patterning using gravure offset printing on an expanded or contracted substrate 40 will be described in detail.
FIG. 5 is a view showing problems that arise when the gate line 4 and the data line 6 of an LCD are fabricated by gravure offset printing. In FIG. 5, the extended direction of the gate line 4 is set as the x direction, and the extended direction of the data line 6 is set as the y direction.
On a liquid crystal panel having x×y area, the gate line 4 and the data line 6 are formed. An insulating layer (not shown) is laminated, and a metal layer is formed at high temperature, thereby expanding the liquid crystal panel Δx toward the x direction. At this time, the expansion toward the y direction will be ignored for convenience' sake. When an ink pattern for forming the data line is formed on the metal layer by gravure offset printing using the cliché and the transfer roll, the first data line 6 is formed at a precise position since it is a reference for the printing. That is, since the transfer roll is operated after positioning the transfer roll on the first data line forming area, the first data line 6 is formed at precise position, and an interval between the gate electrode 3 and the data line 6 is maintained at a set value d.
When the printing continues by advancing the transfer roll, n gate lines 4 are formed, and intervals between the respective gate lines 4 increase as much as Δx/n compared to the original interval. Therefore, when the second data line 6 is formed, the interval between the second data line 6 and the gate electrode 3 is d+Δx/n, and this interval is mis-aligned. The interval between the third data line and the gate electrode is d+2Δx/n, and the interval is increases more and more, and the interval between the nth data line and the gate line is d+Δx.
Generally, the size of pixel in LCD has a magnitude of tens of μm, and the size of TFT is a few μm. On the other hand, the liquid crystal panel may expand more than a few μm by heat processing, although this can vary according to the kind of substrate. Therefore, when mis-alignment is generated between the metal patterns, the TFT can function normally at the transfer roll inlet area (print starting point of the transfer roll) since a fine mis-alignment is generated at the transfer roll inlet area. However, a normally functioning TFT cannot be formed at the transfer roll outlet area (print ending point) since the mis-alignment of Δx (a few μm) is generated at the transfer roll outlet area. Even when the degree of expansion of the liquid crystal panel is larger than the pixel area unit, the nth data line (or source/drain electrode) cannot be formed.